Digital recording of video signals on magnetic tape is the subject of published recommendations provided by the society of motion picture and television engineers (SMPTE). These have become known as the D1 and D2 standards. In the recommended D2 Standard it is specified that the video information of a picture field is to be distributed for recording on two or more successive oblique tracks on the tape. Each oblique track contains one video sector and four audio sectors, each sector beginning with a so-called preamble. After the preamble two or more synchronization blocks follow and then data and checking blocks. The sectors each terminate with a so-called postamble.
The reproduction of video signals thus recorded requires an intermediate memory into which the individual data blocks of the video signal played back from the oblique tracks are written and from which they are again read out for piecing together a complete television picture. The temporary storage of the information permits playback of television pictures both at normal speed and also with every possible slow motion and accelerated motion speed. In reproduction of television pictures with accelerated (time-compression) speed, however, the intermediate memory operates as an intergrator with infinitely long time constant, however, because in this mode of operation the rotating playback magnetic heads scan valid video data only in small track sections, so that older portions of a television picture can be replaced with current video data only after a relatively long time. The stale picture portions produce a "torn up" impression in observation of the television picture.
For mitigation of these disturbing picture effects it has already been proposed in U.S. Pat. No. 4,791,499 to collect the picture information in groups and to write them into a memory with an age indication for a storage duration dependent upon the operation of the tape transport speed. If one of the stored groups overstays the prescribed storage duration, the corresponding group is erased. Then an error concealment stage downstream of the memory can derive error free substitute data in place of the data of the erased picture elements, the substitute data being obtained from originally neighboring error free pixels.
This known method has the disadvantage, however, that a system for administering the storage duration during a picture memory write-read cycle, must carry out several computer operations in order to determine the storage duration for the picture information under consideration. Furthermore, within the picture memory read-write cycle erasing values must be written into the picture memory in the place of data groups found to be stale. The logic components available at present are yet too slow to carry out these operations during the time available therefor. In addition, disturbances during dropouts in the played back data stream lead to additional delays in the determination of the storage duration of the data groups written into the picture memory.